The present invention relates to an array antenna radio communication apparatus and a calibration method.
An array antenna radio communication apparatus is a radio communication apparatus that is provided with a plurality of antennas and enables free setting of directivities by adjusting an amplitude and a phase of respective signals that are received by the respective antennas. The adjustment of the amplitude and the phase with respect to the received signals is carried out by multiplying the received signals by a complex coefficient (hereinafter, the complex coefficient is called a xe2x80x9cweightxe2x80x9d).
The array antenna radio communication apparatus is capable of intensively receiving signals arriving from a desired direction by adjusting the weight to be multiplied. This means that the apparatus has a xe2x80x9creceiving directivityxe2x80x9d. The array antenna radio communication apparatus is able to highly maintain a receiving SIR (Signal to Interference Ratio) of signals arriving from respective directions by having the receiving directivity.
However, in the array antenna radio communication apparatus, the characteristics of the radio receiving circuits, by which the frequencies of the respective signals received through a plurality of antennas are down-converted to base band, are different from each other, depending on unevenness in the characteristics of analog elements such as an amplifier, etc. Therefore, there may be a case where a directivity is formed that is different from a directivity that is expected to be obtained by multiplying by a weight due to the fact that received signals are provided with different unknown amplitude fluctuations and phase rotations.
In order to prevent such situations from occurring, it is necessary to adjust the characteristics that the radio receiving circuits have, so as to be made the same. However, it is remarkably difficult to accurately adjust the characteristics of analog elements such as an amplifier, etc., time-constantly. Accordingly, such a method is employed, in which the characteristics held by the radio receiving circuits each are measured and recorded in a memory without adjusting the characteristics of the radio receiving circuits, and a weight to be used for multiplication is decided by taking it into consideration that the amplitude and phase of the received signals are changed by errors in characteristics. Such a method is called xe2x80x9cCalibrationxe2x80x9d.
There are two types of calibration, one (1) of which is that errors in characteristics of radio receiving circuits are measured in advance by a calibration signal being a known signal before commencement of communications, the results of the measurement are stored in advance in a correction table as characteristic errors to be corrected during communications, and receiving processing is carried out so that errors in characteristics of the radio receiving circuits are counterbalanced by using the permanent correction table during communications, while (2) of which is that, in order to cope with chronological changes in the characteristics of radio receiving circuits, errors in characteristics of the radio receiving circuits are measured from time to time by calibration signals during communications, the results of the measurement are stored from time to time in a correction table as characteristic errors to be corrected, and receiving processing is carried out so that errors in characteristics of the radio receiving circuits are counterbalanced by using the adaptive correction table.
However, the following problems exist in the conventional array antenna radio communication apparatus and calibration method.
That is, 1) in a method in which calibration is performed before commencement of communications, since receiving processing is carried out by using a permanent correction table during communications, the method cannot cope with an amount of phase rotation that changes with the elapse of time and heating of the radio receiving circuits. Therefore, reliability in the results of calibration deteriorates with the elapse of time, thereby deteriorating the performance of the array antenna radio communication apparatus.
On the contrary, 2) in a method in which calibration is carried out during communications, since it is not possible to obtain reliable results of calibration unless there are many samples concerning the results of the measurement, it is necessary to transmit calibration signals a sufficient number of times during communications. But, the greater the number of times of transmission of calibration signals is made, the worse the receiving quality of communication signals becomes because the calibration signals interfere with the communication signals. Also, since there is a limitation in the amount of transmissions in which the calibration signals and communication signals are combined, the greater the number of times of transmission of calibration signals is made, the greater the amount of transmission of communication signals is reduced, whereby the transmission efficiency of user data is lowered. Further, since a sufficient number of samples corresponding to the respective receiving power values are not obtainable if there are a few samples in the results of the measurement, a case occurs where the reliability in the results of the calibration is lowered in the case where the receiving power value of the communication signals changes with the elapse of time. Accordingly, the performance of the array antenna radio communication apparatus may deteriorate.
It is therefore an object of the invention to provide an array antenna radio communication apparatus and a calibration method, by which calibration can be accurately carried out while suppressing interference with communications.
The inventors of the present invention found out that the reliability in calibration can be prevented from being lowered with the elapse of time while suppressing interference with communications, by using both calibration that is performed before commencement of communications and calibration that is performed after commencement of communications while making the functions of the calibrations different from each other, and arrived at the present invention.
Therefore, in order to achieve the above-described object, the present invention utilizes calibration that is performed before the commencement of communications, in order to obtain in advance the amount of correction having high reliability on the basis of a sufficient number of samples before the commencement of communications, and utilizes calibration that is performed after the commencement of communications, in order to maintain reliability in the amount of correction by adjusting the amount of correction on the basis of a small number of samples during communications.